Reconstitution & Dilution
Overview
Many injectable drugs are supplied as a powder because they are unstable in solution. Reconstitution adds a stated volume of a stated diluent — usually sterile water or normal saline — to produce a solution of known strength. The vial label specifies both, and it also states the final concentration, which is not simply the drug divided by the diluent: the powder itself takes up space, so adding 1.8 mL may produce 2 mL of solution. That final concentration is the number that belongs in the dose formula, and once a multi-dose vial is reconstituted it carries a new, shorter expiry.
Interpretation
Technique
Worked example: label says add 1.8 mL, yields 2 mL at 250 mg/mL; order is 500 mg
- Read the labeldiluent, amount, final concentration
- Add 1.8 mL diluentthe stated amount, not a guess
- Roll to dissolvedo not shake
- Use 250 mg/mLthe label's concentration
- Draw 2 mL500 / 250 = 2 mL
During — Monitoring
Patient Teaching
Clinical Pearl
The label tells you the diluent and the final concentration. Use its concentration in the formula — never the volume you added.